The objective of the proposed research is to test the hypothesis that the bone ingrowth into porous plastic coating on femoral prostheses (in dogs) can serve as the attachment vehicle for the cement-free fixation of joint implants. Specific aims include the determination of the change in tissue response (including bone ingrowth) a) with position along the medial and lateral aspects of the prostheses, and b) with implantation time (through 2 years). Thirty porous polysulfone coated cobalt-chrome canine femoral prostheses will be implanted into 30 adult, male English foxhounds (2 yrs. old; 30-35kg). Clinical evaluations will include radiography and radionuclide bone imaging. Postoperatively, the animals will be maintained on an active, quantitated exercise program. Five dogs will be sacrificed at 1, 2, 4, 6, 12 and 24 months. Perfusion fixation of the hind limbs will be performed to insure preservation of tissue structure. Femoral specimens will be allocated for microradiography and ground sectioning, undecalcified Jung Model K microtomy, and decalcification and conventional paraffin techniques. The microscopy sections will be analyzed using histomorphometric techniques. Linear and areal measurements of features in the histological sections will be obtained using a digitizer board/ computer system in conjunction with a drawing tube mounted in a light microscope. Histomorphometric parameters including percent bone, percent osteoblastic activity, percent osteoclastic activity, percent marrow, percent fibrous tissue, percent cellular infiltrates and others will be computed for the tissue within and adjacent to the porosity along the medial and lateral aspects of the prosthesis. Fluorochrome bone labeling will be employed to determine rates of bone turnover. The quantitative histology procedure will allow for the distinction of differences in the tissue response to the prostheses a) at different points along the femoral stem, and b) with implantation time. This data will not only serve to begin to qualify porous coated prostheses for clinical use but also provide a better understanding of bone formation and remodeling behavior. Unoperated femurs will serve as "controls".